Estimation of the Respiratory Syncytial Virus Hospitalization Burden in Older Adults in the Veneto Region, Italy: A Modeling Study.

Introduction: Respiratory syncytial virus (RSV) in older adults can cause a variable spectrum of symptoms, ranging from mild manifestations to hospitalization and sometimes adverse outcomes. However, its true epidemiological burden is underestimated due to non-specific symptoms, lack of standardized diagnostic criteria, limited lab confirmation, and inadequate attribution in administrative datasets.

Methods: We conducted a time-series analysis using hospital discharge data from the Veneto Region, Italy, between 2018 and 2024. Respiratory infections (RI) and RSV-related hospitalizations were identified using International Classification of Diseases codes. A generalized additive mixed model (GAMM) was applied to weekly RI admissions, incorporating circulating pathogen data from the RespiVirNet surveillance system. Seasonal patterns and age-stratified risk were modeled using smoothing terms.

Results: Among individuals aged ≥ 65 years, RSV accounted for an estimated 3.0% to 4.6% of RI hospitalizations. Age-specific hospitalization rates attributable to RSV were 26.8, 109.4, and 317.4 per 100,000 person-years in the 65-74, 75-84, and ≥ 85 age groups, respectively. Explicit RSV coding underestimated the true burden by a factor of up to 7.6. Incidence rates and underreporting were highest in post-acute COVID-19 seasons.

Conclusions: RSV-related hospitalizations in older adults are substantially underreported in administrative data. Improved surveillance and prospective clinical studies are needed to validate model estimates and assess diagnostic test performance. Statistical modeling represents a valid approach to estimate the burden of RSV hospitalizations in underdiagnosed populations, such as the elderly, when direct data are lacking.